1. Field of the Invention
The present invention relates to a digital x-ray detector and its fabrication method and, more particularly, to a digital x-ray detector and its fabrication method capable of preventing a defective contact by strengthening an electrical connection via a contact hole and obtaining reliability of the contact hole.
2. Discussion of the Related Art
Currently, a diagnosing x-ray inspecting device commonly used for medical purposes performs photographing by using an x-ray detecting film and the file is subjected to be printed for a certain time to obtain the results.
However, recently, in line with the advancement of semiconductor technology, a digital x-ray detector (DXD) using a thin film transistor (TFT) has been studied and developed. The digital x-ray detector is advantageous in that results can be diagnosed in real time immediately when x-raying is performed
The structure of the general digital x-ray detector will now be described in detail with reference to the accompanying drawings.
FIG. 1 is a sectional view schematically showing an operation of the general digital x-ray detector.
As shown in FIG. 1, a digital x-ray detector 1 includes an array substrate (not shown) at a lower portion thereof, TFTs 3, storage capacitors 4, charge collecting electrodes 19, a light conductive film 2, an upper electrode 5, high voltage DC power (Ev), or the like.
The light conductive film 2 internally forms an electrical signal, namely, pairs of electrons-holes 7, in proportion to external signal strength such as incident electric waves or magnetic waves. Namely, the light conductive film 2 serves as a converter that converts an external signal, especially, x-ray, into an electrical signal.
In this case, the electron-hole pairs 7 formed by x-ray, gather in the form of electric charges at charge collecting electrodes 19 positioned below the light conductive film 2 by the voltage applied from the high voltage DC power (Ev) positioned at the upper electrode 5 on the light conductive film 2, and stored in the storage capacitor 4 formed together with a storage electrode grounded from the exterior. The charges stored in the storage capacitor 4 are sent to an external image processing element by the TFTs 3 controlled by the exterior, and produce a certain x-ray image.
In this respect, for the digital x-ray detector 1 to detect weak x-ray light and convert it into a charge, the value of the trap state density to trap charges within the light conductive film 2 should be reduced. Also, a high voltage should be applied vertically between the upper electrode 5 and the charge collecting electrodes 19 to reduce current flowing by the voltage other than in a vertical direction.
However, charges within the light conductive film 2 generated by x-ray light are also trapped to gather at an upper portion of a protection layer (not shown) protecting a channel part of the TFT 3 as well as at the charge collecting electrodes 19. The trapped and collected charges induct charges to the channel region at the upper portion of the TFT 3 to cause much leakage current even when the TFT 3 is turned off to make the TFT 3 not perform a switching operation.
In addition, because the electrical signal stored in the storage capacitor 4 flows outside of the storage capacitor 4 due to leakage current in the OFF state, a desired image cannot be properly expressed.
FIG. 2 is a plan view schematically showing a portion of the array substrate of the general digital x-ray detector.
As shown In FIG. 2, the array substrate 10 of the digital x-ray detector includes a gate line 16 and a data line 17 arranged horizontally and vertically to define a pixel area, a TFT, a switching element, formed at the crossing of the gate line 16 and the data line 17, and a ground line 8 are arranged in one direction and commonly grounded with an adjacent pixel.
A storage electrode 9 and a second pixel electrode 18′ forming a storage capacitor as a charge storage unit are formed at the pixel area, and a second insulating layer (not shown) formed of a silicon nitride film as a dielectric material is inserted between the storage electrode 9 and the second pixel electrode 18′.
The charge collecting electrode 19 is formed to extend up to an upper portion of the TFT, electrically connected with the second pixel electrode 18′ via a third contact hole 40c to accumulate holes generated from the light conductive film (not shown) in the storage capacitor, and electrically connected with a drain electrode 23 via a first contact hole 40a, a second contact hole 40b and the first pixel electrode 18 to allow the holes stored in the storage capacitor to be combined with electrons introduced via the TFT.
In this case, the TFT includes a gate electrode 21 connected with the gate line 16, a source electrode 22 connected with the data line 17, and the drain electrode 23 connected with the first pixel electrode 18. In addition, the TFT includes a first insulating layer (not shown) to insulate the gate electrode 21 and the source and drain electrodes 22 and 23, and an active pattern (not shown) formed of an amorphous silicon thin film.
A gate pad electrode 26p and a data pad electrode 27p are formed at edges of the array substrate 10 and electrically connected with the gate line 16 and the data line 17, respectively. The gate pad electrode 26p and the data pad electrode 27p are connected with an outer lead out IC (Integrated Circuit) according to a tape carrier package (TCP) method.
Namely, the data line 17 and the gate line 16 extend in one direction, respectively, so as to be connected with the first and second data pad lines 17p and 17p′ and the gate pad line 16p, and the first and second data pad lines 17p and 17p′ and the gate pad line 16p are electrically connected with the data pad electrode 27p and the gate pad electrode 26p via a fourth contact hole 40d and a fifth contact hole 40e formed at a third insulating layer (not shown).
The process of fabricating the digital x-ray detector basically requires a plurality of masking processes (namely, photolithography processes) to fabricate the array substrate including TFTs, so a method for reducing the number of masks is required in terms of productivity.
FIGS. 3A to 3H are sectional views sequentially showing the fabrication process of the array substrate taken along lines IIa-IIa′, IIb-IIb and IIc-IIc in the digital x-ray detector as shown in FIG. 2.
As shown in FIG. 3A, a first conductive film is formed on the array substrate 10 and patterned by using a photolithography process (a first masking process) to form the gate electrode 21 and the gate line (not shown) at the pixel area of the array substrate 10 and the gate pad line 16p at a gate pad part of the array substrate 10.
Next, as shown in FIG. 3B, a first insulating layer 15a, an amorphous silicon thin film and an n+ amorphous silicon thin film are sequentially deposited on the entire surface of the array substrate with the gate electrode 21, the gate line and the gate pad line 16p formed thereon, and then, the amorphous silicon thin film and the n+ amorphous silicon thin film are selectively patterned by using a photolithography process (a second masking process) to form an active pattern 24 formed of the amorphous silicon thin film at an upper portion of the gate electrode 21.
An n+ amorphous silicon thin film 25, which has been patterned in the same shape as the active pattern 24, is formed on the active pattern 24.
As shown in FIG. 3C, a second conductive film is formed on the entire surface of the array substrate 10 and then selectively patterned by using a photolithography process (a third masking process) to form the source electrode 22 and the drain electrode 23 on the active pattern 24.
Through the third masking process, the data line 17 formed of the second conductive film is formed at the data line region of the array substrate 10 and the first data pad line 17p formed of the second conductive film is formed at the data pad part of the array substrate 10.
Also, through the third masking process, the ground line 8 formed of the second conductive film is formed in the pixel area of the array substrate 10 such that it is arranged in a direction substantially parallel to the data line 17 and commonly grounded with an adjacent pixel.
A portion of the n+ amorphous silicon thin film formed on the active pattern 24 is removed through the third masking process to form an ohmic-contact layer 25n that allows the source/drain regions of the active pattern 24, the source electrode 22, and the drain electrode 23 to be in ohmic-contact.
Thereafter, as shown in FIG. 3D, the storage electrode 9 connected with the ground line 8 is formed at the pixel area of the array substrate 10 through a fourth masking process.
With reference to FIG. 3E, a second insulating layer 15b formed of a silicon nitride film is formed on the entire surface of the array substrate 10 with the storage electrode 9 formed thereon and selectively patterned by using a photolithography process (a fifth masking process) to form a first contact hole 40a exposing a portion of the drain electrode 23 and a pad part hall (H) exposing a portion of the first data pad line 17p. 
As shown in FIG. 3F, a third conductive film is formed on the entire surface of the array substrate 10 with the second insulating layer 15b formed thereon and selectively patterned by using a photolithography process (sixth masking process) to form the first pixel electrode 18 electrically connected with the drain electrode 23 via the first contact hole 40a and the second data pad line 17p′ electrically connected with the first data pad line 17p via the pad part hole (H).
At this time, the second pixel electrode 18′ formed of the third conductive film is formed above the storage electrode 9, and overlaps with a portion of the storage electrode 9 with the second insulating layer 15b interposed therebetween to form a storage capacitor.
With reference to FIG. 3G, a third insulating layer 15c is formed on the entire surface of the array substrate 10 and partially removed through a seventh masking process to form the second contact hole 40b and a third contact hole 40c exposing portions of the first and second pixel electrodes 18 and 18′, respectively. In this case, the third insulating layer 15c is formed of an organic insulating layer to reduce parasitic capacitance.
A portion of the third insulating layer 15c is removed by using the seventh masking process to form fourth and fifth contact holes 40d and 40e, exposing portions of the second data pad line 17p′ and the gate pad line 16p, respectively, at the data pad part and the gate pad part.
As shown in FIG. 3H, a fourth conductive film is formed on the entire surface of the array substrate 10 and selectively patterned through a photolithography process (an eighth masking process) to form the charge collecting electrode 19 electrically connected with the first and second pixel electrodes 18 and 18′ via the second and third contact holes 40b and 40c. 
Through the eighth masking process, the data pad electrode 27p and the gate pad electrode 26p are formed to be electrically connected with the second data pad line 17p′ and the gate pad line 16p via the fourth and fifth contact holes 40d and 40e. 
Although not shown, a process of coating a photosensitive material is performed. The photosensitive material is used as a converter that receives an external signal and converts it into an electrical signal and made of a compound of amorphous selenium.
After the photosensitive material is coated, a transparent upper electrode is formed to allow x-ray light to transmit therethrough.
As described above, the fabrication of the array substrate of the digital x-ray detector including the TFTs requires a total of eight photolithography processes to pattern the gate electrode, the active pattern, the source/drain electrodes, the storage electrodes, the first contact hole, the pixel electrode, the second to fifth contact holes, the charge collecting electrodes, and the like.
In addition, because the array substrate of the general digital x-ray detector includes three insulating layers and three transparent electrode layers, the transparent electrode patterning process is performed three times to form the storage capacitors, causing problems in that the working process is complicated and the gate electrode and the source/drain electrodes are worn out by a transparent electrode etching solution.
Here, the photolithography process transfers a pattern formed on a mask onto the substrate with a thin film deposited thereon to form a desired pattern, which includes a plurality of processes such as a process of coating a photosensitive solution, an exposing process and a developing process, etc, so the plurality of photolithography processes degrade a production yield.
In particular, the masks designed for the pattern is high-priced, so the increase in the number of masks applied to the processes increases the fabrication costs of the digital x-ray detector proportionally.
In addition, the tape automated bonding (TAB) technique is applied to attach the IC for the lead out, for which contact holes are required to electrically connect the lower pad line and the upper electrode.
Also, when the pad lines such as the gate pad line, the data pad line and the storage pad line are formed on the array substrate of the general digital x-ray detector, if metal such as molybdenum which is easily etched by dry etching is applied, the pad lines may be overetched or may not be properly etched due to the dry etching.
Moreover, in case of the general digital x-ray detector, the thickness of the third insulating layer is formed to be relatively thick in order to reduce parasitic capacitance, while the second insulating layer is formed to be relatively thin in order to increase the storage capacitance, resulting in that the thicknesses of the insulating layers at the drain region, the storage region, the storage pad part, the gate pad part and the data pad part where contacts exist are different. The difference in the thicknesses of the insulating layers causes the drain electrode, the ground line and the pad line to be overetched or not to be properly etched in the etching process for forming the contact holes, degrading the reliability of the contact holes.